Changes in Chlorophyll Fluorescence Related to Photoinhibition of Photosynthesis and Cold Acclimation of Green Plants

Abstract

Fluorescence induction at 20oC and 77K was applied in this study to characterize the photoinhibition of spinach leaves (Spinacia oleracea L.) induced in moderate light at chilling temperature. When plants acclimated to +18oC and 260–300 μmol photons m-2 s-1 were exposed to 550 μmol m-2 s-1 at +4oC, they developed a strong photoinhibition within a few hours. This was manifested by a decline in variable fluorescence (FV) of both photosystems (PS) and in quantum yield of O2 evolution. The changes in fluorescence characteristics were more pronounced in the signal recorded at 20°C that at 77K. Moreover, the ratio of variable to maximum fluorescence (FV/FM) measured at 20°C was more closely related to quantum yield than FV/FM at 77K. The FV/FM ratio recorded at 20°C is thus regarded as the more sensitive indicator of photoinhibition. The effects related to photoinhibition of unhardened spinach leaves, including a pronounced increase of initial fluorescence (Fo), were fully reversible at +18°C in dim light. The reversible photoinhibition is viewed as a protective mechanism serving for thermal dissipation of excess light energy. Cold acclimation of the plants increased their resistance to photoinhibition. The mechanism of photoinhibition seemed to be altered in cold acclimated leaves, as indicated by constant Fo. Cold acclimation apparently enforces other protective mechanisms, and photoinhibition then occurs only in higher light.